1. Field of the Invention
The present invention relates to an image signal processing device for a digital still camera or similar imaging apparatus and, more particularly, to an image signal processing device capable of reducing false signals appearing when the high frequency components of luminance signals are produced from signals output from a CCD (Charge Coupled Device) image sensor on which a G stripe, R/B full checker color filter is fitted.
2. Description of the Background Art
A digital still camera, for example, includes an imaging device implemented as a CCD image sensor. A color filter for separating an R (red), a G (green) and a B (blue) color component pixel by pixel is fitted on the image sensor. Various kinds of color filters are known in the art and include a G stripe, R/B full checker color filter. Considering the noticeable influence of G component signals on resolution, the G stripe, R/B full checker color filter has R filter components, G filter components and B filter components arranged such that a number of G component signals can be detected. For example, the filter components are arranged in a repetitive GRGB pattern. In addition, the filter components are arranged such that the G filter components form vertical stripes while the R filter components and B filter components alternate with each other in the vertical direction. In this condition, the R filter components and B filter components each forms a checker pattern.
An image signal processing device for converting the R, G and B color components output from the above image sensor to luminance signals Y and chrominance signals R-Y and B-Y is conventional. It has been customary with this conventional image signal processing device to determine a luminance signal YL, a component YH of the luminance signal YL lying in a high frequency range and chrominance signals R-Y and B-Y for each pixel by using the color component signals of, e.g., a 2 (vertical)×2 (horizontal) pixel matrix including the pixel under observation. Specifically, when the 2×2 pixel matrix including a pixel whose luminance signal should be determined consists of four color component signals R, G1, G2 and B, the luminance signal YL is determined by use of a formula 0.3R+0.295(G1+G2)+0.11B.
Further, the component YH lying in a high frequency range is produced by a formula (G1+G2)/2 if the color component signal of the pixel whose luminance signal should be determined is a G component signal, or by a formula (R+B)/2 if it is an R or a B component signal. The component YH is subtracted from the luminance signal YL. The resulting difference is passed through a low pass filter to turn out a luminance signal YL1 which is a low frequency component. The component YH is added to the luminance signal YL1 so as to output a luminance signal Y implementing a high resolution.
However, the conventional device of the type determining the component of the luminance signal lying in a high frequency range by using two color component signals on two adjoining scanning lines has some problems left unsolved, as follows. When a difference in level between the two color component signals is great at a color boundary between the pixels, a false signal occurs at the color boundary. For example, assume an RB column having the R and B filter components arranged alternately. Then, if a difference in level between the R and B component signals of the adjoining pixels is great, the value of the component YH derived from the formula (R+B)/2 noticeably differs from the original value and turns out a false signal.
Moreover, a false signal ascribable to the fold of the high frequency component of the component YH of the luminance signal causes a vertical stripe to appear in an image. It is a common practice to reduce this kind of false signals by passing the components YH of the luminance signals through a low pass filter so as to reduce the frequency components lying in the high frequency range. This, however, cannot be done without deteriorating the resolution of an image. It is therefore difficult to reduce vertical stripes while guaranteeing a desired resolution.